1. Field of the Invention
The present invention relates to a drilling fluid telemetry system and, more particularly, to a valve for modulating the pressure of a drilling fluid circulating in a drill string in a well bore.
2. Description of the Background
Drilling fluid telemetry systems, referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, salinity, direction and deviation of the well bore, bit conditions and logging data, including resistivity of the various layers, sonic density, porosity, induction self potential and pressure gradients.
In previous borehole telemetry systems, it was first necessary to pull up the drilling pipe section by section including the drilling bit to completely vacate the drilled hole prior to making measurements of important parameters at the bottom of the borehole. Sensors were then lowered down to the bottom of the well on a wireline cable, the measurements were taken, the sensors and wireline were removed and finally the bit and drilling pipe was reassembled and put back into the hole. Obviously, these procedures were extremely expensive and time consuming as a result of the cessation of drilling operations during measurement times.
These problems have led to research in borehole telemetry systems in which the drilling pipe and bit do not have to be removed from the well before measurements are made. Attempts have been made to telemeter data by means of sonic waves traveling through either the drilling pipe or through the drilling mud present both inside and surrounding the drilling pipe. Unfortunately, the drilling mud is a strong sonic dampening medium and substantially attenuates the sonic waves before they can travel a usable distance. Acoustic systems using the drill pipe as the conductor require the use of repeater subs in the pipe string and are electrically complicated. No commercial acoustic system has yet been developed. Total useful telemetry depth with these systems is less than minimally needed in a practical operation.
Other proposed systems have employed an electrical conductor installed either inside or outside the drill pipe or the casing pipe. Unfortunately, the physical forces encountered in a borehole drilling operation inside the well bore and the cuttings and other debris brought up from the bottom of the well bore often produce malfunctions in the conductors and associated electrical connectors.
Another proposed system utilizes a conductor inside of each section of drill pipe with transformer coupling between sections of pipe. Besides requiring expensive modifications to the drill pipe, these systems are unreliable because the magnetic coupling between sections if frequently hindered by mechanical misalignment between drill pipe sections and because the alignment of coupling coils with one another is difficult to achieve.
Still other proposed systems employ either the drilling pipe or casing pipe as one of the conductors in an electrical transmission system. The earth itself may form the other conductor. Unfortunately, the conductivity of the earth is unpredictable and is frequently too low to make this system practical at typical borehole depths. Still further, these systems often include a single wire along the casing pipe or drilling pipe. These systems suffer from the problems discussed above with the hard wire systems. Both of these systems suffer the additional common problem that the conductivity between pipe sections is greatly affected by the presence of contaminants on the pipe joints. Frequently the resistance of the pipe joints is too high to permit telemetry using any practical power levels.
Still other proposed systems involve various electromagnetic transmission schemes for directing electromagnetic signals up the pipe string to the surface, either through the pipe or mud. These systems, similar to the sonic systems discussed above, are complicated by attempts to overcome the attenuating affects of these transmitting mediums.
At present the only drill string telemetry systems which have achieved commercial success are those related to mud pulse telemetry. One example of such a prior mud pulse system is illustrated in U.S. Pat. No. 3,964,556 which requires that circulation of drilling fluids be ceased in order to operate the system. Other systems have used a controlled restriction placed in the circulating mud stream and are commonly referred to as positive pulse systems. With mud volume sometimes surpassing 600 gpm and pump pressures exceeding 3000 psi, the restriction of this large, high pressure flow requires very powerful downhole apparatus and energy sources. Further, these systems must deal with the movement of valve parts under such high pressure conditions, resulting in a source of problems dealing with the durability of valve parts subjected to high pressure, abrasive, fluid flow conditions.
A presently employed mud pulse system involving negative pressure pulse techniques includes a downhole valve for venting a portion of the circulating drilling fluids from the interior of the drill string to the annular space between the pipe string and the borehole wall. As drilling fluids are circulated down the inside of the drill string, out through the drill bit and up the annular space to the surface, a pressure of about 1000 to about 3000 psi is developed across the drill bit. Thus, a substantial pressure differential exists across the wall of the drill string above the drill bit. By momentarily venting a portion of the fluid flow out a lateral port above the bit in the drill string, a momentary pressure drop is produced and is detectable at the surface to provide an indication of the downhole venting. A downhole instrument or detector is arranged to produce a signal or mechanical action upon the occurrence of a downhole detected event to produce the above-described venting. As may be readily appreciated by those skilled in the art, the sophistication to which this signalling may be developed is practically unlimited.
A major problem associated with negative pressure pulse systems is the wear and replacement of valve parts, particularly as the data rate is expanded. It is highly desirable to operate such a system as long as possible since replacement of system components typically requires the time consuming and expensive removal of the valve system from its downhole location and from the drill string at the surface.
One negative pulse system uses a poppet valve having a circuitous flow path through the valve. The seat of the poppet is worn rapidly by high rates of abrasive fluid flow when the valve is in the open position. In addition, it is desirable to have a fast acting opening and closing movement of the valve parts in order to create a sharp pressure pulse for adequate detection at the surface. Rapid closing of the poppet valve generates a high valve head impact force on the seat. This force rapidly wears the valve parts, particularly when abrasive particles are present in the fluid flow through the valve. Such particles become impacted in the valve parts and deteriorate the sealing surfaces of the valve. The repeated impact forces may also break portions of the valve parts because erosion resistant materials are generally not impact resistant.
Another negative pulse system employs a rotary acting valve which as a result of the mass of rotary valve parts and the motor system used to operate the valve is a slow acting system.
These examples illustrate some of the crucial considerations that exist in the application of a rapidly acting valve to a fluid flow to generate a sharp pressure pulse. Other considerations in the use of these systems in borehole operations involve the extreme impact forces and vibrational forces existing in a drill string application and resulting in excessive wear and fatigue to operating parts of the system. The particular difficulties encountered in a drill string environment, including the requirement for a long-lasting system to prevent premature malfunction and replacement of parts require a simple and rugged valve system.
The present invention overcomes the foregoing disadvantages and provides a new and improved mud pulse telemetry system having an improved shear type valve gate which is simple, durable, efficient and conveniently serviceable.